1. Field of the Invention
The present invention relates to an exposure apparatus, a measurement method, a stabilization method, and a device fabrication method.
2. Description of the Related Art
A projection exposure apparatus has conventionally been used when fabricating fine semiconductor devices such as an LSI and ULSI by using the photolithography technique. In this projection exposure apparatus, a projection optical system transfers a circuit pattern formed on a reticle (mask) onto a substrate such as a wafer by projecting the circuit pattern onto the substrate.
Recently, as demands for micropatterning of semiconductor devices increase, the projection exposure apparatus is beginning to require high resolution, and attempts are being made to obtain a short wavelength of exposure light and a high numerical aperture (NA) of the projection optical system (i.e., to increase the NA of the projection optical system). When increasing the resolution, however, the depth of focus decreases, and this makes it necessary to increase the accuracy of focusing to match the surface of a wafer with the image formation plane (focusing plane) of the projection optical system.
Also, in the projection exposure apparatus, the accuracy of alignment that accurately aligns circuit patterns transferred in a plurality of steps is one important characteristic. Since the fluctuation in magnification of the projection optical system largely influences this alignment accuracy, it is extremely important to maintain the magnification of the projection optical system at a predetermined value (constant value).
On the other hand, when irradiated with exposure light, the projection optical system absorbs part of the energy of this exposure light. Heat generated by this absorption changes the temperature, and the optical characteristics (e.g., the magnification and aberration) of the projection optical system change. If the projection optical system is kept irradiated with the exposure light, therefore, the optical characteristics of the projection optical system fluctuate, and the focusing accuracy or alignment accuracy may decrease (i.e., a non-negligible focusing error or alignment error may occur).
Accordingly, Japanese Patent Publication No. 63-16725 has proposed a technique that corrects the fluctuations in optical characteristics of the projection optical system caused by irradiation with the exposure light. In this technique disclosed in Japanese Patent Publication No. 63-16725, the fluctuations (fluctuation amounts) of optical characteristics of the projection optical system caused by irradiation with the exposure light are calculated by a model expression using the exposure amount, exposure time, non-exposure time, and the like as parameters, and the fluctuations in optical characteristics of the projection optical system are corrected based on the calculation result.
The above-mentioned model expression has a coefficient (correction coefficient) unique to the projection optical system for each optical characteristic of the projection optical system. To calculate this correction coefficient, it is necessary to measure the fluctuation in optical characteristic by actually irradiating the projection optical system with the exposure light. The application of the exposure light to the projection optical system performed for purposes, such as the calculation of the correction coefficient, other than intended exposure (i.e., exposure for transferring a reticle pattern onto a wafer) will be referred to as “dummy exposure” hereinafter.
Dummy exposure may be performed for purposes other than the calculation of the correction coefficient. For example, dummy exposure is performed to stabilize the transmittance of the projection optical system. When emission and pause of the exposure light (e.g., an excimer laser) is repeated, quartz, which is generally used as the glass material of the projection optical system, abruptly changes the transmittance due to the emission time and pause time of the exposure light. More specifically, as shown in FIG. 9, the transmittance of the projection optical system temporarily decreases immediately after the emission (exposure) of the exposure light is started, starts increasing from a certain point, and saturates at a certain value when the exposure light emission continues. It is very difficult to correct this abrupt change in transmittance. As disclosed in Japanese Patent Laid-Open No. 10-116766, therefore, it is effective to stabilize the transmittance of the projection optical system by performing dummy exposure of about tens of thousands of pulses before starting (resuming) exposure. FIG. 9 is a graph showing the change in transmittance of the projection optical system as a function of the emission time (exposure time) of the exposure light. Referring to FIG. 9, the ordinate indicates the transmittance of the projection optical system, and the abscissa indicates the emission time of the exposure light.
The exposure light of dummy exposure passes through almost the same optical path as that of the exposure light of intended exposure. Accordingly, if dummy exposure is performed with a wafer being placed on a wafer stage (i.e., with a wafer being set in the exposure position), the wafer is exposed and becomes unusable as a product, so the exposure light of dummy exposure must irradiate an object other than the wafer. Therefore, an object (for dummy exposure) to be irradiated with the exposure light of dummy exposure is fixed on the wafer stage, and set in the exposure position when performing dummy exposure.
When fixing the object for dummy exposure on the wafer stage, however, a space for fixing the object must be secured, and this upsizes the wafer stage.
Also, in dummy exposure for calculating the correction coefficient, the object for dummy exposure preferably has the same reflectance as that of a wafer for fabricating an actual product, in order to measure the fluctuation in optical characteristic of the projection optical system in intended exposure. However, the object for dummy exposure generally has a reflectance and shape different from those of a wafer. Therefore, the fluctuation in optical characteristic of the projection optical system in dummy exposure differs from that in optical characteristic of the projection optical system in intended exposure. This makes it difficult to accurately calculate the correction coefficient. This is so because if the intensity of reflected light from the object for dummy exposure differs from that of reflected light from a wafer, an optical characteristic fluctuation different from that in intended exposure occurs.
In dummy exposure for stabilizing the transmittance of the projection optical system, the transmittance of the projection optical system can be stabilized more efficiently (i.e., within a shorter time) when the amount of light returning to the final plane (the plane closest to a wafer) of the projection optical system is larger. Accordingly, the object for dummy exposure must have a highest reflectance. This requirement does not match the requirement for the reflectance of the object for dummy exposure for calculating the correction coefficient.
The above problem can be solved by fixing a plurality of objects for dummy exposure different in reflectance on the wafer stage. However, if the number of the objects for dummy exposure increases, the area of the space to be secured on the wafer stage increases.
Also, when the object for dummy exposure fixed on the wafer stage is irradiated with the exposure light for a long time, the object raises the temperature of the wafer stage, and mechanisms (e.g., a holding mechanism for holding a wafer) of the wafer stage thermally deform.
Furthermore, even when the object for dummy exposure is made of a material having a resistance to the exposure light such as an excimer laser, the surface of the object deteriorates and changes its properties if the object is irradiated with the exposure light for a long time, so the object must be exchanged. However, the object for dummy exposure is fixed on the wafer stage as described previously, and this makes the work of exchanging the object difficult and cumbersome.